diff --git a/contrib/check-code.py b/contrib/check-code.py
--- a/contrib/check-code.py
+++ b/contrib/check-code.py
@@ -73,6 +73,7 @@ testpats = [
     (r'^stop\(\)', "don't use 'stop' as a shell function name"),
     (r'(\[|\btest\b).*-e ', "don't use 'test -e', use 'test -f'"),
     (r'^alias\b.*=', "don't use alias, use a function"),
+    (r'if\s*!', "don't use '!' to negate exit status"),
   ],
   # warnings
   []
diff --git a/tests/test-check-code-hg.t b/tests/test-check-code-hg.t
--- a/tests/test-check-code-hg.t
+++ b/tests/test-check-code-hg.t
@@ -1,6 +1,7 @@
   $ check_code="$TESTDIR"/../contrib/check-code.py
   $ cd "$TESTDIR"/..
-  $ if ! hg identify -q > /dev/null; then
+  $ if hg identify -q > /dev/null; then :
+  > else
   >     echo "skipped: not a Mercurial working dir" >&2
   >     exit 80
   > fi